The present invention relates to the measurement of moisture in materials which broadly could be considered as dielectric in their propeties. More specifically, it relates to the determination of moisture by measurement of the impedance of the dielectric material at differing alternating frequencies. Provision is made for correcting errors introduced because of temperature variation and variation in the nature of the dielectric material itself. The method is particularly well adapted for measurement of the moisture content of wood.
It is important to control the moisture content of many materials having dielectric properties. Grain can serve as one example. If the moisture content of stored grain is too high, mold growth and ultimate spoilage can result. Wood is another material in which moisture should desirably be controlled within specified limits which depend upon the ultimate application. Wood intended for construction framing should desirably have a moisture content below 20% to minimize fungal attack. Finish lumber, such as that used for trim and moldings, normally is specified with moisture contents at or near those which will be in equilibrium with the ultimate environment. This is typically in the 7-10% range. Wood which is to be adhesively bonded or laminated is normally dried to moisture contents of 12% or below.
Through the years, a number of nondestructive moisturemeasuring methods have been developed. Most of these depend upon some electrical property of the material being measured. The earliest instruments for measuring the moisture content of wood determined its electrical resistivity by passing a direct current through it. Handheld moisture meters were soon developed so that moisture content could be readily estimated in either the manufacturing plant or in the field where the product was being used. These normally comprise a pair of pins or needles of predetermined size and spacing which are driven into the wood along the grain direction. The needles are connected to a direct current source and a megohmeter, which is precalibrated to give a direct estimate of moisture content. Similar meters have been developed for measurement of the moisture in many other materials.
Another type of meter was developed somewhat later that did not depend on making direct contact with the material. These are typically capacitively coupled. They are suitable for use on conveyor lines in a manufacturing plant, and are often used in conjunction with ancillary equipment which can mark or reject high moisture samples for later redrying. Internal impedance is the property most commonly measured. In many cases the internal resistance is by far the most important component of the impedance. Thus, many of the noncontact meters are similar to the direct-current meters in their use of resistivity to estimate of moisture content.
In the case of wood, resistivity is an almost ideal parameter to measure because of its great variation with relatively small differences in moisture content. The resistance of Douglas-fir at 27.degree. C., using needle electrodes 3.1 cm apart and driven into a depth of 0.8 cm, drops from about 22,400 megohms at 7% moisture content to 0.60 megohms at 24% moisture. Accuracy below this moisture range begins to fall off because of the difficulties in measuring very high resistances. Accuracy also is decreased as the fiber saturation point of the species is approached. No satisfactory instrumental method is yet in use for accurate estimation of moisture content of solid wood members above the fiber saturation point.
The noncontact moisture meters vary considerably in their mode of operation. The most common ones for measuring the moisture content of a dielectric material capacitively couple the material into one arm of a bridge circuit. The bridge unbalance is then measured as two alternating frequencies are impressed across the bridge, either simultaneously or sequentially. These alternating current signals are then filtered into the original component frequencies and rectified to produce DC analog signals. The resultant voltages are a function of the ratio of change in voltage drop across the test capacitor which correspond to each frequency, when the material being tested is located between the plates of the capacitor. Most typically, the estimated moisture content is calculated from the analog voltages by dividing the higher frequency component by the lower frequency component with the inclusion of appropriate constants. Exemplary meters of this type are shown in the patents to Davidson U.S. Pat. No. 3,155,899; Walls U.S. Pat. No. 3,155,902; Baird U.S. Pat. No. 3,241,062; and Liu U.S. Pat. No. 3,255,412.
In U.S. Pat. No. 3,155,902, Walls notes a number of deficiencies in capacitively-coupled moisture meters. A number of these relates to the internal stability and calibration of the electronic component. Two others are a result of uncontrolled outside influences. Walls notes that the measurement is not independent in the position of material between the capacitor plates. He further notes that the measured moisture content has a temperature dependency. However, the inventor offers no solutions for either of these problems. Perry, in U.S. Pat. No. 3,339,137 and 3,354,388, shows a noncontact meter that overcomes the position problem by using opposed electrodes having a fieldfree region between them. His electrodes are at equal voltage and polarity. This system provides compensation for positioning and it is essentially immaterial where the dielectric is located in the void space between the electrodes,
Baird, in U.S. Pat. No. 3,241,062, shows a relatively complex system of temperature compensation. This involves a sensor and associated circuitry which uses a servomotor to adjust a series of potentiometers controlling the output voltage of one of the oscillators. A major problem with this system is the lag time associated with electromechanical system.
One problem appears to be as yet unaddressed. A given moisture meter is normally calibrated so as to work only on a specific dielectric material. Even in the measurement of wood moisture content there is uncompensated variation from species to species. Meters are normally calculated on the basis of coastal Douglas-fir. If, for example, a meter so calibrated is used on pine or hemlock, somewhat different moisture readings will be indicated even though the moisture content of all samples is identical.
The present ivention comprises a method for measuring the moisture content of dielectric materials which has an internal electronic compensation for the temperature and nature of the dielectric material being measured. It is of the general type which employes a plurality of alternating current signals of different frequencies impressed across a bridge circuit into which the sample material is capacitively coupled.